The present invention relates generally to the manufacture of containers and more particularly involves a process and apparatus for thermoforming containers from thermoplastic polymers such as polypropylene.
In conventional thermoforming systems, one preferred method for forming containers involves the pressure/vacuum method of drawing heated thermoplastic sheet material into a mold and forming it against the mold wall by differential pressure.
It has been known that particular thermoplastics such as polypropylene have a comparatively long cooling cycle and therefore jacketed mold cavities having a liquid cooling capacity can speed up the thermoforming cycle. Such jacketed molding systems usually comprise a female mold cup having a double-walled construction allowing for the circulation of a coolant such as water around the exterior of the mold cup to speed up withdrawal of heat from the thermoformed container. Although jacketed mold cups have significantly reduced thermoforming times by speeding up the polymer cooling cycle, the cycle is still relatively long and a need has existed for means of shortening the cycle.